1. Field of the Invention
The present invention relates to full color displays. More particularly, this invention pertains to an improved liquid crystal display of the type that is illuminated by both ambient and artificial light.
2. Description of the Prior Art
At present, full color displays for use in aircraft and military vehicles commonly utilize a cathode ray tube (CRT). While such displays provide adequate resolution and brightness, they are relatively bulky, consume much power in operation and often require cooling systems that further contribute to the bulk and the power consumption of the total system. These factors are often of critical importance to the design of airborne systems.
The drawbacks of full color CRT displays have led to investigation of the feasibility of full color displays based, inter alia, upon the use of a layer of electrooptic liquid crystal material. Liquid crystal technology offers the possibility of flat, relatively thin and therefore highly compact construction. Further, the voltages and power levels required to actuate liquid crystal materials to a preferred orientation for modulating incident light are relatively low.
The principal classifications of liquid crystal material are twisted nematic, guest-host (or Heilmeier), phase change guest-host and double layer guest-host. The particular liquid crystal material employed dictates the type of optical modulation that is effected by the light valve. For example, twisted nematic material causes the polarization of the light passing therethrough to become reoriented (usually by ninety degrees). Guest-host materials, so-called by the presence of a dye that aligns itself with the liquid crystal molecules, modulate light as a consequence of the property of the dye to absorb or transmit light in response to the orientation of the molecules of the liquid crystal. In phase change guest-host materials, the molecules of the liquid crystal material are arranged into a spiral form that blocks the majority of the light in the "off" state. The application of a voltage aligns the molecules and permits the passage of light. A double layer guest-host liquid crystal comprises two guest-host liquid crystal cells arranged back-to-back with a ninety degree orientation between the molecular alignment of the two cells.
Liquid crystal displays may be arranged to operate in a transmissive mode, a reflective mode, or both. Generally, the reflective mode is most suitable for operation under high ambient light conditions while the transmissive mode, requiring backlighting, is most usefully employed in applications involving dark and low ambient (e.g. office) lighting conditions. The combination of both modes of operation is known as the "transflective" mode of operation. This mode is particularly-appropriate for broad range operation which includes, of course, many important applications including, for example, the environment of the cockpit.
Presently, liquid crystal displays for operation in the transflective mode comprise a liquid crystal light valve in combination with a source of artificial visible backlighting. A light diffusion plane is located intermediate the light source(s) and the light valve. The optical properties of the plane generally represent a compromise between the needs to (1) transmit artificial backlight and (2) reflect ambient light, when available, through the liquid crystal light valve. The state of the liquid crystal material is spatially controlled by a transparent electrode matrix. (When an active device such as a thin film transistor or "TFT" is incorporated into the array to enhance the addressability of the matrix, it is known as an active matrix display.) Pixel locations are addressed and the molecules of the adjacent liquid crystal material are modulated so that a spatial distribution of pixel areas of preselected varied light transmission is attained. In a full color display, a full color filter that comprises a matrix or mosaic of individual (narrow band) primary color filters provides the sole means for selecting the desired color band of the white light (transmitted backlighting and reflected ambient light) that passes through the liquid crystal material layer. Light absorbing dyes form the (red, green and blue) color elements of the matrix. The color elements of the filter are in registration with the elements of the electrode matrix so that, by addressing the electrodes in accordance with the color coding of the filter matrix, the transmission of white light through the liquid crystal material produces a full color dislay. A representative display of this type is disclosed in U.S. patent Ser. No. 3,840,695 of Fischer for "Liquid Crystal Image Display Panel With Integrated Addressing Circuitry."
Displays in accordance with the above-described typical arrangement are beset with many difficulties. The use of absorbing dye filters to color the image passing through the liquid crystal layer represents a highly inefficient usage of the energy contained in the light source. Each dedicated filter element essentially blocks the transmission of two thirds of the white light through the valve. That is, in the prior art, each pixel is illuminated with white light, requiring that color filters transmit only the desired portion of the white light spectrum while absorbing all other wavelengths. In total, about one third of the energy from the white light source is transmitted through each of the filter "windows".
This energy absorption effect is even more dramatic with respect to the utilization of (reflected) ambient light by the display. Reflected light must pass through the filter twice, multiplying the energy loss. As a result, the display is often unsuitably dim. Correction for such dimness often involves an increase in the power level of the backlighting that, in turn, introduces additional undesirable effects. In addition to conflicting with the goal of a low energy display, this may produce harmful temperature rises within the system.
The design of a single filter for coloring both backlight and ambient illumination is further complicated by the different chromaticity effects experienced by the reflected light which passes twice through the filter and the transmitted light that passes only once through that filter. This disparity can result in a display of differing hues during high and low ambient lighting conditions.
Another deficiency of the conventional approach is that, in the desire to minimize undesirable parallax effects, the color filter elements must be closely located immediately adjacent the liquid crystal layer. Thus, the diffusion (or back) plane, which must be behind the liquid crystal layer, is recessed by the thickness of the glass layer at the rear of the liquid crystal. The resultant spacing of the filter and the somewhat-reflective backplane can produce two deleterious effects during reflective mode operation of the display. Ambient light will generally include off-axis rays. When ambient light is absorbed in the liquid crystal layer, it creates a shadow on the diffusing plane along the axis of the incident light. Therefore, as the display is viewed at an angle with respect the direction of such incident light, the image appears to be displaced with respect to the image created in the liquid crystal layer. This results in an annoying double image or "shadowing" effect. Of perhaps even greater significance, the spacing of the backplane from the color matrix can produce cross-contamination between the primary colors of the filter. This results when an oblique, off-axis ray of ambient light passes through a filter of one primary color when incident upon the front of the light valve and then exits the valve through a filter of a second primary color after reflection from the backplane.